Until recently, it was believed that female gonads of most mammalian species, including humans, house a finite number of meiotically-arrested germ cells (oocytes) enclosed within primordial follicles that serve as the stockpile of eggs released at ovulation during each menstrual cycle (Gougeon, A. et al, (1996) Endocr Rev. 17: 121-55; Morita, Y. & Tilly, J. L., (1999) Dev. Biol. 213: 1-17). Oocyte numbers decline throughout postnatal life, though mechanisms involving apoptosis (Tilly, J. L., (2001) Nat. Rev. Mol. Cell Biol. 2: 838-848), which were widely believed to eventually leave the ovaries barren of germ cells (Faddy, M. J. et al., (1976) J. Exp. Zool. 197: 173-186; Faddy, M. J. et al., (1987) Cell Tissue Kinet. 20: 551-560; Faddy, M. J., (2000) Mol. Cell Endocrinol. 163: 43-48). In humans, exhaustion of the oocyte reserve typically occurs during the fifth decade of life, driving menopause. (Richardson, S. J. et al. (1987) J. Clin. Endocrinol. Metab. 65: 1231-1237).
According to this basic doctrine of reproductive biology, it was further believed that once depleted, the ovarian germ cell pool could not be replenished. (Zuckerman, S. (1951) Recent Prog. Horm. Res. 6: 63-108; Borum, K., (1961) Exp. Cell Res. 24: 495-507; Peters, H., (1970) Phil. Trans. R. Soc. Lond. B, 259: 91-101; McLaren, A., (1984) Symp. Soc. Exp. Biol. 38: 7-23; Anderson, L. D. and Hirshfield, A. N. (1992) Md. Med. J. 41: 614-620). Thus, any treatment that accelerates the loss of oocytes threatens to decrease the fertility and will cause menopause at an earlier age than expected. For example, exposure of women to a wide spectrum of agents that damage the ovary, such as chemotherapeutic agents and radiotherapy, generally leads to premature menopause and irreversible sterility. At present, the limited therapeutic options of preserving fertility and normal ovarian function under various adverse conditions are invasive, such as for example cryopreservation of ovarian tissue fragments or single oocytes, and often require hormonal therapy, which can be medically inappropriate for many women with hormonally responsive tumors (Waxman, J. (1983) J. R. Soc. Med. 76: 144-8; Familiari, G. et al., (1993) Hum. Reprod. 8: 2080-7; Ried, H. L. & Jaffe, N., (1994) Semin. Roentgenol. 29: 6-14; Reichman, B. S. & Green, K. B. (1994) J. Natl. Cancer Inst. Monogr. 16: 125-9). In addition, there are currently no therapeutic options for postponing normal ovarian failure at menopause. Therefore, there is great need in the art for further discovery and development of new or less invasive therapeutic interventions for restoring failed ovarian function and infertility in women.